Permanent magnet speed detector

ABSTRACT

This invention provides an improved magnetic angular speed detector in which an annular magnet having a number of magnetised sectors of alternate polarity rotates, with a member the speed of which is to be detected, relative to an annular coil formed with a number of loops, preferably as a printed circuit of zig-zag configuration. The circumferential width of the loops of the coil is equal to the circumferential spacing between like pole sectors of the annular magnet.

United States Patent 1 Giachello [4 1 Mar. 12, 1974 PERMANENT MAGNETSPEED DETECTOR Marco Giachello, Turin, Italy Assignee: Fiat Societa PerAzioni, Turin, Italy Filed: July 27, 1972 App]. No.: 275,666

Inventor:

[30] Foreign Application Priority Data Aug. 2, 1971 Italy 69596/71 US.Cl 310/156, 310/168, 310/268 Int. Cl. H02k 21/12 Field of Search310/168, 768, 156, 207,

References Cited UNITED STATES PATENTS 1/1966 Henry-Baudot 310/268 XMattingly 310/156 Raymond 310/268 Primary Examiner--D. F. DugganAttorney, Agent, or Firm-Sughrue, Rothwell, Mion, Zinn & Macpeak [57]ABSTRACT 1 Claim, 5 Drawing Figures PAIENTEDMAR 12 I974 SHEET 1 [IF 2III FIG. 1

PAIENIEBIIARIZIQM 3.796 899 sum 2 0F 2 FIG. 4

PERMANENT MAGNET SPEED DETECTOR BACKGROUND OF THE INVENTION Thisinvention relates to magnetic angular speed detectors, more particularlyspeed detectors having permanent magnets and arranged to generateelectrical signals having a frequency, voltage and/or pulse widthproportional to the rotational speed of a member such as a shaft.

Previously known angular speed detectors generally comprise a coilresponsive to variations of a magnetic field associated with therotating member to provide a voltage proportional to the speed ofrotation of the member. In one known speed detector the coil is soplaced as to detect variations in the magnetic field due to a permanentmagnet rotating with the rotary member.

In such earlier known detectors the permanent magnet is magnetised in agenerally radial direction with respect to the axis of rotation of themember with which it rotates, and in view of the inevitable irregularityand spreading of the magnetic field some inaccuracy is introduced andthis, combined with any eccentricity in the mounting of the rotarymember, gives rise to the possibility of errors in the detected signal.In view of this high accuracy is essential in the mounting of the staymember, and expensive bearings are necessary in order to avoid variationin the relative positions of the interacting parts of the speed detectordue to wear of the bearings.

Moreover, this known detector suffers from the disadvantage that thedetecting coil is affected at any given instant only by a small part ofthe magnetic field of the magnet, so that the electrical signalgenerated is necessarily small.

An object of this invention is, therefore, to provide an angular speeddetector device of the permanent magnet type which is simple tomanufacture, of low cost, and which is relatively insensitive toalignment inaccuracies between the parts, eccentricity or wear, andwhich generates speed signals with high efficiency.

Another object of the invention is to provide a simple process for themanufacture of the permanent magnet used in the angular speed detector.

SUMMARY OF THE INVENTION According to the invention there is provided amagnetic annular speed detector device comprising a magnet arranged torotate with a rotating member the speed of which is to be detected andan electrical winding relative to which the magnet rotates to induce avoltage in the-winding as a' result of the magnetic flux variationstherein, wherein the improvement consists in that the magnet is formedby a number of sectors magnetised with alternate polarity and arrangedin an annulus coaxially with the rotating member and in that the windingcomprises a single annular coil constituted by a meandering zig-zagconductive path formed on an annular surface facing the magnetic annulusand coaxial therewith, the loops formed by the zigzag path having acircumferential width equal to the circumferential spacing of adjacentsectoral pole pairs of said magnet.

The invention also provides a process for the manufacture of a speeddetector device as aforesaid, said process including the steps ofclamping the annular magnet, initiallymagnetised with a uniformmagnetisation, relative to the annular coil, rotating the said magnetand the coil together, detecting thepassage of successive loops of theannular coil past a fixed point, and inverting the magnetisation of themagnet by energising an electromagnet while one half of thecircumferential width of each said loop passes said point, to effect themagnetisation of the annular magnet in sectors.

Alternatively, the process may comprise rotating the annular magnet,initially magnetised with a uniform magnetisation, and subjecting saidannular magnet to an alternating magnetic field at a frequency equal toan integral multiple of the frequency of rotation of the annular magnetso as to achieve magnetised sectors of alternate polarity and of thedesired circumferential width.

BRIEF DESCRIPTION OF THE DRAWINGS A preferred practical embodiment willnow be described, merely by way of example, with reference to theaccompanying drawings, wherein:

FIG. 1 is a diagrammatic axial sectional view of an angular speeddetector device according to one embodiment of the invention;

FIG. 2 is a cross-sectional view in the direction of the arrows II-II ofthe speed detector device of FIG. 1;

FIG. 3 is a cross-sectional view in direction of the arrows llI-III ofthe speed detector device of FIG. 1;

FIG. 4 is a schematic drawing of an apparatus for carrying out a processfor manufacturing the permanent magnet of the speed detector device asshown in FIG. 2, according to another aspect of the invention, and

FIG. 5 is a schematic drawing of another apparatus for magnetising thesaid permanent magnet by a process according to the invention.

FIG. 1 shows disgrammatically in axial section a speed detector deviceaccording to this invention in which a chassis of a car or other vehicleconstitutes a fixed support 10 and carries an annular disc 12 ofnonmagnetic material, for example a synthetic material. On its faceremote from the support 10 the disc 12 carries a thin annular plate 14formed of a material of low magnetic reluctance, for example of sinteredsteel or ferrite. To the plate 14 there is applied a further annularplate 16 of insulating material which carries a printed circuit (notshown in FIG. 1) on its face 17 remote from the plate 14. This printedcircuit will be described further with reference to FIG. 2.

A rotating member 18 of which the angular speed is to be detectedcarries at an end adjacent the fixed support 10 a supporting disc 20 towhich an annular permanent magnet 22 is attached. The annular magnet 22is formed of plasto ferrite material, magnetised as described later withreference to FIG. 3. The annular plate 16 and the annular magnet 22 havesubstantially equal inner and outer radii and are mounted coaxially witheach other.

FIG. 2 shows diagrammatically part of the printed circuit carried by theannular plate 16. This printed circuit is formed by a continuousmeandering or zig-zag conductive track 34 on the face 17 of the plate16, the opposite ends of the track 34 being provided with terminals 36and 38 located adjacent each other. The track 34 has, therefore,radially extending conductive parts 39, 41, 43, which are alternatelyinterconnected at their radially inner and radially outer ends by shortcircumferentially extending conductive parts 40, 42, and 44, 46,respectively. The pitch of the zig-zag track 34, or the mean distancebetween corresponding radial parts of the track, such as 39 and 43, isuniform for the entire annular track.

With reference to FIG. 3, the annular magnet 22 is magnetised in adirection parallel to its axis, in a number of sectors 26, 28, 30, 32,of alternate polarity. The mean circumferential width of each sector isequal to half the pitch of the zig-zag track 34 on the annular plate 16.

Consequently, when the annular magnet 22 and the zig-zag track 34 arecounterposed in axial alignment, as shown in FIG. 1, the substantiallyaxial magnetic field produced by the magnet 22 and linked with the track34 will vary between two maxima of opposite polarity (north, N andsouth, S). Thus supposing one sector 26 of north magnetism (N) isdirectly opposite to the area delimited by the track parts 41, 46, 43 onthe inside of the track 34, then sectors of North polarity (N) will alsobe located directly opposite all the other analogous areas of the track34, while the sectors of south polarity (S) will be located oppositeareas which are on the outside of the zig-zag track 34. By rotating themagnet 22 with respect to the track 34 through one half-pitch of thetrack 34 the opposite situation will result, in which all the Southpoles (S) will, in effect, be inside the track 34 and all the Northpoles (N) will be outside the track 34.

It will be seen, therefore, that by rotating the shaft 18 with respectto the fixed support 10, the magnetic flux of the annular magnet 22links with the zig-zag track 34 and the frequency of the variation ofthe flux linkage, and, therefore, the induced voltage in the track 34,is proportional to the rotational speed of the shaft 18.

By using a material of high remanence for the annular magnet 22, such asa plasto-ferrite, and a material of low reluctance for the annularsupport plate 14 of the printed circuit it is ensured that the lines ofthe magnetic field due to the magnet 22 are directed substantiallyperpendicular to the plane of the track 34, with only slight spreadingor divergence of the field direction.

By making use of an axially directed magnetic field as compared with aradially directed field, and by arranging that at each instant the wholesurface of the annular magnet 22 co-operates with the track 34, theelectromotive force'induced between the terminals 36 and 38, being thesum of the electro-motive forces induced in each single loop of thetrack, is significantly higher than in earlier known devices of thiskind. As a result the detector according to this invention is relativelyinsensitive to eccentricity, misalignment or other variations ofrelative positioning of the relatively moving parts.

For example the case will be considered where there is a slightinclination and axial misalignment of the plane of the magnet 22 withrespect to the plane of the track 34. The magnetic flux linked with thetrack 34 in the zone in which the distance between the magnet and thetrack is at a maximum will be reduced, and the flux linking thediametrically opposite zone of the track will be increased. Since,however, the electro-motive force generated at the output terminals 36,38 of the track is, as stated previously, given by the sum of theelectromotive forces generated in each loop of the track over the wholecircumferential extent of the track, the resultant voltage across theterminals 36, 38 will not be subject to periodic fluctuations as theshaft 18 rotates through 360.

The insensitivity of the device to positioning errors is due also to thesubstantial linearity of the magnetic field lines, which remainperpendicular both to the magnet 22 and to the track 34, that is,parallel to the common axis of the magnet 22 and the annular plates 14and 16. This allows manufacture of the device with ample tolerance bothin the relative positioning of the axes of the relatively moving parts14, 22, and inthe spacing between the magnet 22 and the track 34.Moreover, the said insensitivity also allows the use of rotationalsupports of low cost since considerable wear of the supports can betolerated before the detector device becomes unusable.

In fact the only constructional aspect in which a high degree ofdimensional precision is required is in the exact correspondencerequired between the spacing of the polar sectors of the annular magnet22 and the spacing of the loops of the track 34. To achieve such exactcorrespondence, this invention also provides a simple process of anapparatus for the sectoral magnetisation of the annular magnet 22, whichfacilitates the manufacture quickly and at low cost of annular magnetsindividually magnetised to correspond to respective printed circuits onrespective annular plates 16.

MAGNETISATION OF THE ANNULAR MAGNET Referring to FIG. 4, the annularmagnet 22, uniformly magnetised in an axial direction, and theannularplate 16 carrying a printed circuit in the form of the conductivezig-zag track 34 on the face 17 are mounted coaxially and non-rotatablyon respective supports shown diagrammatically as comprising a rotarymandrel 50 and a counterpoint 52. The annular magnet 22 and the printedcircuit plate 16 are suitable clamped between respective pairs of rigiddiscs 54, 56, and 58, 60.

An electro-magnet 62 having a magnetising coil 64 is provided with twopolepieces 66, 68 the faces of which are parallel to each other andelongated radially with respect to the axis of the annular magnet 22.The circumferential width of the polepieces 66 and 68, in a directionperpendicular to the plane of FIG. 4, is slightly smaller than thedesired circumferential width of the pole sectors of the annular magnet.

The coil 64 of the electro-magnet 62 can be connected to a directcurrent source 70, of high power, through a relay switch 72. Themagnitude of the current which fed to the coil 64 is so calculated thatthe magnetic field generated between the polepieces 66, 68 when therelay switch 72 is closed is sufficient to invert completely themagnetisation of the annular magnet 22 in the Zone of the latterincluded between the polepieces.

Two sliding contacts 74, 76, aligned on a common radius with respect tothe common axis of the magnet 22 and of the annular plate 16, aredisposed in a fixed position and in contact with the face 17 of theplate 16 on which the printed circuit track 34 is provided. The contact74 is connected to a direct current supply 78, while the contact 76 isconnected to the input of a bistable multivibrator 80, having a singleinput, to cause commutation of the multivibrator each time a circuit iscompleted between the contacts 74 and 76, that is, when a radial part ofthe track 34 is located between the contacts. The output line 82 of thebistable multivibrator 80 is connected to and controls the energisationof the winding 84 of the relay switch 72.

To magnetise the annular magnet 22, the mandrel 50 is rotated, causingrotation of both the printed circuit plate 16 and the magnet 22 itself.It will be supposed that initially the output voltage of the bistablemultivibrator 80 on the line 82 is zero: the relay switch 72 istherefore open, and the magnetisation of the sector of the magnet 22between the polepieces 66 and 68 will therefore remain unchanged.

As soon as the contacts 74 and 76 engage a radial part of one of theturns of the zigzag track 34, commutation of the bistable multivibrator80 occurs, causing an energising voltage to be applied to the relaywinding 84, and thereby closing the relay switch 72. The polepieces 66and 68 now create in the sector of the magnet located therebetween aninstantaneous magnetic induction which inverts the magnetisation of themagnet 22 in said sector. This reversing magnetic induction ismaintained after the two contacts 74 and 76 have ceased to make contactwith the said radial part of the track 34: the bistable multi-vibrator80 is returned to its initial state when the two contacts engage thenext adjacent radial part of the track 34, whereupon the relay switch 72reopens, switching off the magnetic field generated by theelectro-magnet 62, so that the next following sector of the annularmagnet 22 will have its initial magnetisation left unchanged.

It will be apparent that, when the mandrel 50 has performed one completerotation of 360, the annular magnet 22 will be magnetised in a number ofpolar sectors of alternate polarity, the angular distribution andcircumferential width of the sectors corresponding exactly with theannular zig-zag printed circuit path 34 on the plate 16.

Naturally the rotational speed of the mandrel should be sufficiently lowto ensure that the effects of the mag netisation transitions and thefrictional drag of the magnetic field generated by the electro-magnet 62on the annular magnet 22 do not reach such a magnitude as to causeanomalies in the regularity of the magnetisatron.

The technique and apparatus illustrated in FIG. 5 therefore enables theannular magnet to be made simply and economically with accuratereference to the annular zig-zag track 34 on the annular plate 16.

FIG. 5 illustrates schematically an apparatus for effecting the sectoralmagnetisation of the annular magnet 22 by means of a different method.An annular plastoferrite plate 22, permanently magnetised in an axialdirection, is fixed to a shaft 86 of a synchronous electric motor 87 bymeans of two discs 88, 90 clamped on opposite faces of the plate 22. Anelectromagnet 92 having a magnetising winding 94 has two parallelradially elongated pole faces 96, 98 the circumferential width of whichis negligible compared with the circumferential width of the desiredmagnetised sectors. The annular plate 22 is located between the polefaces 96, 98 and the winding 94 is energised by a power amplifier 100which amplifies an oscillatory voltage provided by an oscillator 102, ofany suitable type, but of high stability.

To effect sectoral magnetisation of the annular magnet 22, the motor 87is rotated at a low angular speed n, while the oscillator 102 provides asinusoidal output signal of frequencyf(in the same units of measurementas the angular speed n) such as to satisfy the relation:

where k is a whole number, equal to the required number of pole pairs onthe annular magnet 22. The electro-magnet 92 imparts the requisitesinusoidal magnetisation to the ring 22, with a wavelength, or pitch,measured circumferentially, equal to the distance between like polesegments on the resulting magnet 22.

The requisites for correct functioning of the apparatus of FIG. 5 areperfect stability both of the rotational speed of the shaft 86 and ofthe frequency of the signal generated by the oscillator 102, and,moreover, precise control of the relation between these two quantities.In particular, the power of the motor 87 must be such that therotational movement of the annular magnet 22 is not affected appreciablyby variations of frictional couples imposed on the magnet by themagnetic field due to the electro-magnet 92, during rotation of themagnet 22.

Naturally, both the angular speed detector device herein described, andthe process and apparatus for the magnetisation of the annular permanentmagnet 22, are given herein merely by way of example, and it will beappreciated that numerous variations are possible both in the structureof the detector device and in the magnetisation process. For example,the permanent magnet 22 could be made in a material other thanplastoferrite, and could also be of composite construction, obtained byplacing side-by-side a number of elementary sector-shaped magnets, eventhough in practice such a manufacturing technique is less advantageousthan that described herein. Also the conductive zig-zag track 34,instead of being aprinted circuit, could be formed by a conductive stripor wire folded into the requisite shape.

It would also be possible, within the scope of this invention, to formthe annular magnet 22 by different methods.

I claim:

1. A vehicle having a chassis constituting a fixed support of thevehicle, a member rotatable with respect to said support, and means,mounted part on said chassis and part on said rotating member, toprovide an electrical signal proportional to the speed of rotation ofthe member with respect to said support, wherein the part of said meansmounted on the chassis comprises an annular disc of non-magneticmaterial fixed to said chassis, a first annular plate of material of lowmagnetic reluctance secured to the face of said disc remote from saidsupport, a second annular plate of insulating material secured to saidfirst plate and carrying a printed circuit remote from the first plate,said printed circuit being formed by a continuous conductive zigzagtrack having a plurality of equally spaced radially extending partsalternately connected at the radially inner and outer ends bycircumferentially extending parts, the pitch of the zig-zag track beinguniform and there being provided adjacent output terminals at theopposite ends of the track, said disc and said plates being mountedcoaxially with said rotating member, and wherein the part of said meansfixed to said rotating member adjacent to the fixed support, and anannular magnet mounted on said supporting disc coaxially with therotating member and counterposed to said track, said second plate andsaid magnet having substantially equal inner and outer radii and saidmagnet being fomied of a number of radially extending sectors magnetizedin a sense parallel to the axis of the rotating member, the individualsectors having a mean circumferential width equal to half the pitch ofthe zig-zag track with adjacent sectors being of opposite polarity.

1. A vehicle having a chassis constituting a fixed support of thevehicle, a member rotatable with respect to said support, and means,mounted part on said chassis and part on said rotating member, toprovide an electrical signal proportional to the speEd of rotation ofthe member with respect to said support, wherein the part of said meansmounted on the chassis comprises an annular disc of non-magneticmaterial fixed to said chassis, a first annular plate of material of lowmagnetic reluctance secured to the face of said disc remote from saidsupport, a second annular plate of insulating material secured to saidfirst plate and carrying a printed circuit remote from the first plate,said printed circuit being formed by a continuous conductive zigzagtrack having a plurality of equally spaced radially extending partsalternately connected at the radially inner and outer ends bycircumferentially extending parts, the pitch of the zig-zag track beinguniform and there being provided adjacent output terminals at theopposite ends of the track, said disc and said plates being mountedcoaxially with said rotating member, and wherein the part of said meansfixed to said rotating member adjacent to the fixed support, and anannular magnet mounted on said supporting disc coaxially with therotating member and counterposed to said track, said second plate andsaid magnet having substantially equal inner and outer radii and saidmagnet being formed of a number of radially extending sectors magnetizedin a sense parallel to the axis of the rotating member, the individualsectors having a mean circumferential width equal to half the pitch ofthe zig-zag track with adjacent sectors being of opposite polarity.